calendar - Why do we need a separate celebration for Shushan Purim in walled cities?

Purim is celebrated on Adar 14 because that was the day the Jews rested after prevailing against their enemies. In walled cities, Shushan Purim is celebrated on the 15th because that was when the fighting ended in Shushan (a walled city).

Later, in deference to Jerusalem, the Sages determined that Purim would be celebrated on Adar 15 in all cities which had been enclosed by a wall at the time of Joshua's conquest of the Land of Israel. This criterion allowed the city of Jerusalem to retain its importance for Jews, and although Shushan was not walled at the time of Joshua, it was made an exception since the miracle occurred there. _Wikipedia

I have 2 questions:

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  Why do we need a separate celebration for walled cities at all?

  
  


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  How does celebrating Shushan Purim in Jerusalem "allow it to retain its importance"?

  
  
  

Answer

The Beis Yosef in Orach Chaim 688:3 addresses both of your questions:

1) "Why do we need a separate celebration for walled cities"

ואיכא למידק למה חלקו מצוה זו לימים חלוקים וקבעו יום מיוחד לפרזים ויום מיוחד לכרכין משא"כ בשאר המצות

Why did they divide this mitzvah [Purim] into separate days, fixing a distinct day for the unwalled cities and a distinct day for the walled cities, unlike any other mitzvah?

ותירץ הר"ן שמפני שהיהודים היושבים בערי הפרזות והיהודים שבשושן לא נחו מאויביהם ביום א' שהרי היהודים הפרזים נחו בי"ד והיהודים שבשושן כחו בט"ו ועשו כל אחד ביום מנוחתו משתה ושמחה לפיכך כשקבעו עליהם י"ט לדורות קבעוה לכל א' ביום שנח בו

The Ran answers that the Jews in the surrounding cities and the Jews in the walled cities didn't rest from fighting on the same day- Jews in the surrounding cities stopped fighting on the 14th, Jews in walled cities stopped fighting on the 15th- and on the day they each rested, they each made a party and had a feast. Thus when they established Purim for generations, they established it for each day that people rested.

Ie- the separate celebration (Shushan Purim) was enacted for this reason.

2) How does celebrating Shushan Purim in Jerusalem "allow it to retain its importance"?

To answer this question, we need to establish why the criteria for celebrating Shushan Purim is based off of cities that were walled during the time of Yehoshua bin Nun vs cities that were walled during the time of Achashveirosh.

וי"ל דהיינו טעמא כדי לחלוק כבוד לא"י כדאיתא בירוש' ופירוש הענין שכיון שהוצרכו לחלוק בין מוקפין לשאין מוקפין כשם שנחלקה שושן משאר עיירות אילו תלו הדבר מימות אחשורוש היתה א"י שהיתה חרבה באותם הימים נדונית כפרזים והיה גנאי בדבר ולפיכך תלו הדבר בימות יהושע בן נון

And there are those who say the reason is in order to give honor to Eretz Yisrael like we see in the Yerushalmi. And explaining the matter that since we needed a distinction between walled vs unwalled cities, just as Shushan was separated from other cities, if we based the the requirement from walled cities during the times of Achashveirosh, it would be an insult to the Land of Israel which then lay in ruins. Therefore the distinction of walled vs unwalled is based off the days of Yehoshua bin Nun.

The Beis Yosef seems to suggest that the fundamental reason why Purim was separated into two days was to honor of the Land of Israel.

Thus celebrating Shushan Purim in Jerusalem is explicitly fulfilling the very purpose of why Shushan Purim was enacted- ie allowing Jerusalem to retain its importance.